This invention relates generally to magnetic disk drives and more particularly to an improved emergency head unload system for use therein.
Modern moving head magnetic disk drives generally employ heads carried by a support structure coupled to a positioner motor. The positioner motor typically includes a coil mounted within a magnetic field for linear movement and oriented relative to the disk to move the heads radially over the disk surface to thereby enable the heads to be positioned over any annular track on the surface. The heads are designed to actually fly above the disk recording surface at heights of less than 100 microinches. If, during normal operation of the drive, power is lost causing the disk rotational speed to gradually decrease, the heads cannot continue to fly and would ultimately crash into the disk surface. In order to protect the data, the heads and the disk, it is necessary to remove the heads from the disk surface as fast as possible when a power fault is detected. The process of removing the heads from the disk in an emergency situation is referred to as an emergency unload procedure and requires that the head support structure be moved radially toward the disk outer track to axially move the heads away from the disk surface. Although loss of power is probably the primary reason for initiating the emergency unload procedure, the procedure is typically also initiated when the following conditions are encountered:
(1) Disc speed does not remain within tolerance;
(2) Positioner error is detected;
(3) Write circuit faults that could effect stored data are detected.
Essentially all modern disk drives incorporate some system for executing an emergency unload procedure in order to avoid loss of data and prevent disk and/or head damage. In a typical prior art emergency unload system, a capacitor is charged by the drive power supply during normal operation. Upon the detection of an emergency condition, a relay or equivalent switching means switches the capacitor across the positioner coil terminals to provide the electromotive force necessary to move the head support structure across the disk surface. Upon approaching the disk outer edge, the head support structure encounters a mechanical ramp. The mechanical ramp imparts an axial force to the support structure thus unloading the head from the disk.
The capacitor typically supplies a relatively constant voltage across the positioner coil causing the coil and support structure to accelerate as it is moving toward the disk outer edge. As a result, the support structure sometimes contacts the mechanical ramp at a high velocity. The resulting impact can cause the head to oscillate and impact against the disk surface thus causing damage to the disk and head.